Cause Several organisms can cause lower stem and root rots on these holiday cacti including Fusarium oxysporum, Pythium spp., and Phytophthora sp. The OSU Plant Clinic has found Fusarium.
Fusarium survives in the soil as thick-walled, dormant chlamydospores, which germinate in response to exudates from nearby plant roots. Hyphae then penetrate the roots, colonize the cortex, and move into the xylem tissue. The fungus can attach wounded or non-wounded plants. Small spores, microconidia, are produced and carried up into the plant.
Both Pythium and Phytophthora are oomycetes and are strongly favored by abundant moisture especially in poorly drained potting media. They can survive in dust, planting media, or soil particles on greenhouse floors, flats, and pots. Wood benches can also be a reservoir. Spread can also occur via movement of infected plants. Damaged plants are more easily infected. Pythium can enter roots damaged by high-soluble salts, either from over-fertilization or from letting the rooting medium dry out, even for short periods, during propagation.
Symptoms
Fusarium - Water-soaked lesions first occur with reddish-brown margins. The tissue then dries to a paper-thin tan. Upper portions of the plant wilt when cut off from functioning roots.
Pythium - Feeder roots become brown and soft rotted where the cortex easily separates from the vascular system. The plant wilts in later stages of disease development.
Phytophthora - Plants collapse and show brown lesions at the base of the plant.
Cultural control
- Do not overwater plants. Allow pots to dry out in-between waterings.
- When stimulating flower bud formation by moving to areas that are cooler and darker (12 hours or more), plants may need less water.
- Provide good drainage for plants in containers.
- Avoid reusing pots or trays from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Remove all dead and dying plants from the production area being careful not to contaminate healthy plants or areas.
Chemical control Focus on cultural controls first. Labels may still list old Zygocactus name. Rotate among fungicides from different groups with different modes of action.
To control Fusarium only:
- Empress at 1 to 3 fl oz/100 gal water can be used for cuttings or seedlings. Group 11 fungicide. 12-hr reentry.
- Heritage at 0.2 to 1 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 6 to 12 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Orkestra at 8 to 10 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Although Broadform was found to be effective it is not allowed for drench application and this plant is not on the label.
To control Rhizoctonia only:
- Cleary's 3336 EG at 8 oz/100 gal water as a drench after transplanting. High rates in combination with mancozeb may damage plants. Group 1 fungicide. 12-hr reentry.
- Heritage at 0.2 to 1 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Medallion WDG at 1 to 2 oz/100 gal water. Use with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
To control Pythium and/or Phytophthora only:
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Heritage at 0.2 to 1 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- MetaStar 2E at 0.5 to 2 fl oz/100 gal water at transplanting, and see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Monterey Garden Phos at 2 to 4 teaspoons/gal water as a foliar spray. Also labeled for soil drench, see label for details. Can be used in landscape sites. Group P7 fungicide. H
- Subdue MAXX at 0.5 to 1 oz/100 gal water at transplanting. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
Notes: chlorothalonil inhibits root growth.
Reference Wick, R.L. 2018. Diseases of Holiday Cacti: Schlumbergera and Hatiora. In McGovern, R.J. and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.